Organization, Basis of Presentation and Liquidity - Narrative (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2021 USD ($) divestiture program segment	Dec. 31, 2020 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of late-stage development programs | program	2
Number of operating segments | segment	1
Total cash, cash equivalents and short-term investments, estimated fair value	$ 393,282	$ 505,109
Number of discrete business divestitures | divestiture	2
Accumulated deficit	$ 1,439,366	$ 1,324,840